1. Field of the Invention
The present invention relates to a back scattered electron detector used together with an X-ray detector included in a charged particle beam apparatus and a charged particle beam apparatus including this back scattered electron detector.
2. Description of the Related Art
When performing an X-ray element analysis using a charged particle beam apparatus typified by a scanning electron microscope, a secondary electron detector and a back scattered electron detector are used in addition to an X-ray detector. A back scattered electron detector (also referred to as “BSE detector”) is disposed between an objective lens and the surface of a sample. Using the secondary electron detector and the back scattered electron detector, an analysis position can be optionally determined while acquiring an image. The X-ray detector is disposed at an angular position different from those of the secondary electron detector and the back scattered electron detector to detect and analyze characteristic X-rays.
In a sample surface analysis using the scanning electron microscope, the surface of a sample to be observed is normally not a completely flat surface, but includes many projections and depressions. For this reason, when there are obstacles such as projections and depressions resulting from the sample surface shape and surface roughness between the X-ray detector and the analysis position on the sample surface, characteristic X-rays generated from the analysis position are blocked by the projections and depressions and the spectral intensity obtained by the X-ray detector decreases. Furthermore, when there is a significant difference in the energy of a detected spectrum (e.g., Al—K and Cu—K), the decreasing rate of the spectral intensity on the low energy side increases and a peak intensity ratio is detected differently.
Therefore, when there are projections and depressions on the analysis surface, it is not possible to perform an accurate analysis in locations hidden behind the projections and depressions when viewed from the X-ray detector. Moreover, when a normal X-ray element analysis is performed, an analysis range is determined while viewing a back scattered electron image from which a composition contrast is obtained in addition to a secondary electron image obtained by a secondary electron detector. However, it is difficult to distinguish locations where the spectral intensity decreases from a back scattered electron image obtained by the back scattered electron detector disposed right above the sample surface. When performing an X-ray element analysis using such a method, an analyzer determines an analysis range by estimating the concavo-convex shape of the sample surface from the secondary electronic image obtained by the secondary electron detector including shape information and evaluates the analysis result obtained from experiences. If there are parts where it is judged that X-rays cannot be detected due to projections and depressions in the analysis range or that a decrease of intensity of X-rays affects analysis data, it is necessary to readjust a positional relationship between the sample and the X-ray detector and recollect (rework) data. Note that if the positional relationship between the sample and the X-ray detector cannot be readjusted, there is no choice but to consider the analysis result by taking into account the concavo-convex condition of the sample surface from the analysis data obtained, and accurate data interpretation may not be possible.
A technique described in WO2012/016198 proposes to dispose a back scattered electron detector in the vicinity of an X-ray detector and eliminate misalignment between an X-ray detected image and a back scattered electron image.
The present inventor et al. discovered a method of disposing, when there are projections and depressions on an analysis surface, a back scattered electron detector in the vicinity of an X-ray detector, using a back scattered electron image detected by the back scattered electron detector, and determining beforehand in which region of a sample the X-ray detector can acquire an accurate X-ray detection image and perform an appropriate analysis. In the process of examining an apparatus suitable for such an analysis technique, the inventor et al. discovered that enabling a back scattered electron detector to be mounted on an existing X-ray detector would significantly expand an application range of this analysis technique.
The present invention has been implemented in view of the above-described problems, and it is an object of the present invention to provide a back scattered electron detector that can be mounted on an existing X-ray detector and suitable for implementing a method for determining beforehand in which region of a sample the X-ray detector can obtain an accurate X-ray detection image and perform an appropriate analysis. Furthermore, a charged particle beam apparatus provided with the back scattered electron detector is also provided.